(1) Field of the Invention
This invention relates to a flat plate having excellent light transmittance, reduced moisture absorption and enhanced mechanical strength, and which is thin and becomes colored only to a negligible extent.
This flat plate is useful as a light guide plate and a light diffusion panel.
(2) Description of the Related Art
Characteristics such as a high light transmittance, a reduced thickness and a reduced distortion due to moisture absorption are required for light guide plates used as a liquid crystal base plate and a light diffuser panel. To provide a light guide plate having the required characteristics, for example, an injection-molded plate has been proposed in Japanese Unexamined Patent Publication No. H7-118344, which is made of a cycloolefin polymer having a norbornane ring structure, made by metathesis-catalyzed polymerization of 8-methyl-8-methoxycarbonyltetracyclododeca-3-ene, and having an intrinsic viscosity of 0.2 to 1.5 dl/g imparted thereto for enhancing the fluidity upon molding. However, this proposal has problems such that, (1) for enhancing the fluidity to provide a thin plate, when the molecular weight of the cycloolefin polymer is reduced, the mechanical strength of the plate is reduced, or the plate is cracked upon molding or becomes brittle, and, when the molding temperature is elevated, the molded plate becomes colored, and further that, in contrast, (2) when the molecular weight of the cycloolefin polymer is increased enough for improving the mechanical strength, the melt fluidity is decreased and a thin plate is difficult to obtain.
In view of the foregoing, a primary object of the present invention is to provide a flat plate such as a light guide plate, which has excellent light transmittance, reduced moisture absorption and enhanced mechanical strength, and which can be thin and becomes colored only to a negligible extent when molding is conducted at a high temperature.
In accordance with the present invention, there is provided a flat plate made of a cycloolefin polymer comprising at least 30% by weight, based on the cycloolefin polymer, of repeating units (A) having an alicyclic structure; said repeating units (A) comprising at least 30% by weight, based on the repeating units (A), of repeating units (A-i) having no norbornane structure; and said cycloolefin polymer having a weight average molecular weight of 5,000 to 50,000.
In accordance with the present invention, there is further provided a light guide plate made of the above-mentioned flat plate.